The present invention generally relates to air purification systems for use in aircraft and, in particular, to environmental control systems (ECS) for use on aircraft to remove pollutants from the ambient air for cabin usage.
A commercial aircraft is generally equipped with an environmental control system (ECS) which provides fresh, conditioned air to the on-board passengers. A typical ECS receives compressed air, or bleed air, from an aircraft gas turbine engine and delivers it to the cabin. This bleed air after expansion has a temperature in the range of 200°-500° C. It is typically directed to a primary heat exchanger, or precooler, where it is further cooled to a temperature in the range of 100°-150° C. From the precooler, this warm bleed air is then sent to an air conditioner that performs a final cooling function and delivers fresh air to the aircraft cabin. A remediation system, such as a stand-alone catalytic converter, is usually interposed between the precooler and the air conditioner, to remove the pollutants from the bleed air from the gas turbine engine which may affect passengers' safety and comfort level.
One such pollutant is ozone, which is present at high concentration levels in the atmosphere at altitudes of 20,000 feet or more. Ozone, even at low levels of concentration, will cause irritation of the respiratory systems of passengers and must therefore be removed. A stand-alone catalytic converter for the destruction of ozone is generally installed to convert over 90% of O3 to oxygen. This stand-alone catalytic converter, though effective, generally requires additional hardware, such as monolithic substrate, shells and pipes, which results in an increase of weight, volume and pressure drop, all of which are undesirable in an aircraft ECS application. Another type of pollutant consists of low level hydrocarbon fumes which occasionally enter the ECS system through the air intake. Hydrocarbon fumes cause odor in cabin air that therefore must be abated for passenger's comfort. To catalytically decompose hydrocarbon fumes into harmless carbon dioxide and water usually requires a reaction temperature above 200° C. The operating temperature of the stand-alone catalyst unit placed downstream of precooler is generally too low to accomplish this.
One approach that addresses these problems is to integrate the catalytic pollutant removal function of the stand-alone catalytic converter with the cooling function of the precooler to form a catalytic precooler. The catalytic precooler combines the functionality of heat transfer and pollutant destruction in a single unit that reduces the weight, volume and pressure drop for the ECS. The precooler, being the primary heat-exchanging device, receives air at higher temperatures than the air received by the downstream stand-alone catalytic converter, but higher gas temperatures are preferred for the catalytic destruction of both ozone and hydrocarbon pollutants. U.S. Pat. No. 4,665,973, to Limberg et al. and U.S. Pat. No. 5,151,022, to Emerson et al. both describe such devices.
Nevertheless, such catalytic converters have problems. First, the incoming air is directed through channels having a pollutant-destroying catalyst interposed. Under high flow space velocity, the incoming air tends to assume a laminar flow along the inner surfaces of the channel and thus has limited interaction with the catalyst on the surface of the wall. The design of the catalytic precooler should ensure sufficient mass transfer between the gas phase and the catalyst-coating surface without causing substantial increase of pressure drop. This is particularly important for a tubular precooler where, because of laminar flow, mass transfer is often insufficient to achieve catalytic destruction levels in excess of 90% for pollutants. Second, the catalytic coating should have high activity; increased tolerance to thermal shock; increased resistance to particle abrasion; increased resistance to deactivation by gas phase catalyst poisoning and dust; and long service life. Finally, the catalytic coating should have sufficient heat transfer capability in order to maintain the cooling function of the precooler.
Thus, as can be seen, there is a need for a catalytic precooler for use in an aircraft ECS which weighs less than combination of a separate precooler and catalytic converter of prior art systems; has better catalytic activity, thermal shock resistance, and particle abrasion resistance than prior art systems; and exhibits an improved mass transfer between the gas phase and the catalyst-coating surface to achieve catalytic destruction levels of pollutants in excess of 90% without causing substantial increase of pressure drop.